Preparation of high quality uranium carbide



Ufl d See P en 9 3,392,005 PREPARATION OF HIGH. QUALITY URANIUM CARBIDE Lloyd A. Hanson, Canoga Park, Calif., assignor, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed June 8, 1967, Ser. No. 645,570 2 Claims. (Cl. 23-349) ABSTRACT OF THE DISCLOSURE Process for preparing high density, low oxygen content uranium carbide by using a nickel additive during carbothermic reduction of uranium oxide to uranium carbide. 1

' BACKGROUND OF THE INVENTION The invention relates to a carbothermic reduction process for reducing uranium oxide to uranium carbide. -In particular, the invention relates to the use of nickel powder as a fluxing agent in the reaction mixture during the carbothermic process.

This invention arose in the course of or under Contract No. AT(1l1)-GEN8 with the United States Atomic Energy Commission.

Uranium carbide is well known as a fuel for nuclear reactors, especially of the fast or gas-cooled type. However, such. fuels arenormally enriched in fissile material and as such are often prepared from reprocessed fuel either of the oxide or carbide type. The carbothermic process is onestep in such reprocessing, and is an alternative to solvent extraction or melt-refining. Essentially, for spent fuel of the oxide type,'the carbothermic process is one of reduction, but if the original spent fuel is a carbide, the process may include an oxidation step prior to the reduction to provide an oxide starting material.

The problem involved in the preparation'of satisfactory UC nuclear reactor fissile fuel-forms are several- First is the problem of excessive oxygen content. Second is the problem that the carbide fissile fuel attack steel cladding at high temperatures. Third is the problem of obtaining sufficient bulk density to permit compact of the fuel and subsequent sintering. Fourth is the problem of providing a sufficiently low finishing temperature. Most important, of course, is the over-all goal of providing a fuel which operates satisfactorily under reactor conditions.

SUMMARY OF THE INVENTION It is an object of this invention to provide improved fuel and the process therefor.

Another object is to provide fuel having low oxygen content,

Still a further object is to provide a low temperature fuel finishing process.

The invention describes an additive fiuxing agent for use in the carbothermic process which therewith more eifectixely and completely reduces uranium oxide to uranium carbide of improved properties. The product has a low oxygen content, large grain size and has high density which allows subsequent greater compaction and permits finishing of the fuel at a lower temperature than heretofore. The process includes the addition and uniform mixture of -10% nickel powder by weight with a blended mixture of uranium oxide U0 or U 0 or U0 and carbon powders. The resulting uniform mixture is then heated in a vacuum to a temperature in the range of about 1400-1500 C. for several hours to produce the carbide, after which the temperature is raised to the range of about 1600-1750 C. for several more hours for finishing the reaction and for vaporizing the nickel agent from the reaction mixture. An optimum amount of nickel is 4-5% by weight. The carbon content may be varied widely depending upon whether hypo, hyper or stoichiometric UC is desired, and is of the order of by weight with respect to uranium.

DESCRIPTION OF PREFERRED EMBODIMENT The carbothermic reduction method of preparing UC as previously practiced is described in reports NAA-SR- 7511 and NAA-SR-6976. This previous method is useful as a step in reprocessing spent fuel of both UC and U0 Type. In reprocessing UC, a prior step of oxidation is included, usually in the same furnace as for the subsequent conventional reduction step.

Essentailly the carbothermic method as heretofore practiced involves reduction of U0 and carbon powder in a vacuum furnace having an induction heater at l300-2100 C. One reaction is UO +3C UC+2CO. The oxide and carbon and thoroughly and uniformly mixed, whereupon they are placed in a graphite or molybdenum crucible for heating to reaction temperature. Subsequently, the resulting UC product may be fabricated by arc-melting and casting, or by pressing into compacts and sintering to provide a solid fuel.

The oxide starting materials involved may be derived from spent fuels which have been reprocessed, or may include virgin oxide fissile fuel material, as desired. However, while such materials are enriched in uranium, they obviously include contaminants and an excess of oxygen.

.Conventional methods of reprocessing fuel may involve solvent extraction, melt-refining or oxidation-reduction processes. In the oxidation-reduction processes, the spent fuel is declad, oxidized to U 0 reduced to U0 using hydrogen for carbothermic reduction to carbide, and then arc-melted and cast into new fuel.

The making of successful and acceptable fuel, of course, includes many factors including cost and other conventional criteria. Most important, however, are the starting materials and whether the final product works well in a reactor environment.

As to the starting materials, the problem is one of contaminants. As to the final product requirement, it is desired to produce UC having a low oxygen content, high density, and a low finishing temperature.

As improved fuel is provided by the process of the invention which solves the problems presented. In particular, the improved fuel is provided by the use of nickel as a fluxing additive during the carbothermic process. More specifically, finely divided nickel additive in the form of powdered nickel metal or nickel oxide is added to the reagent mixture of powdered U0 and carbon in amounts in the range of 0.5% to 10% by weight. The optimum amount of nickel is from 4-5 by weight. The carbon/ uranium oxide weight ratio may be varied widely depending upon whether hyper, hypo or stoichiometric UC is desired.

The mixture of powdered U0 carbon and nickel additive is placed in a vacuum induction furnace. The temperature within the oven is raised to 1400-1500 C. as a first heating step for several hours (usually 2-4 hours), whereupon the carbon reacts with the metallic oxide to produce a carbide product with CO being released. 'Substantially, the oven temperature is raised to 1600-1750 C. for several more hours (usually 2-4 hours) as a second heating step for finishing the UC product and for vaporizing the neckel and removing any excess oxygen.

One reason for the improved product by the process above is that nickel melts at 1455 C. Thus, during the first heating step, the nickel melts and acts as a flux in the carbothermic reaction. The nickel also reduces the temperature required for the finished product and promotes grain growth. Thus during the tirst heating step, a liquid phase exists in the carbothermie process.

During the subsequent second heating step. .b- 1750 C., the nickel is vaporized to remove some or the residual oxygen.

This process has been used in making UC and tests on the finished products have shown the following characteristics. The UC contained from 50-45% carbon by weight and from 100-380 parts per million oxygen when using a nickel input of from 245% by weight. The resulting UC pellets had a geometric density or 9.6 to l2.4 grams/co, and where particles were sized to simulate the input for vibratory compaction, the UC particle density was from 13.0 to 13.4 grams/cc. The residual nickel in the UC particles was less than parts per million.

In comparison with other known UC tuels. this product as stated above has very low oxygen content. a high density and very small remaining nickel. Because or the low finishing temperature, there is very little variation in the UC composition.

The equilibrium pressure of CO over a UO -C mixture is expressed as Since the carbothermic reduction reaction LS normally carried out in a vacuum chamber where the ultimate or the pumping system is about to 10- ttmosoheres. that portion of the reaction near the end. :.e.. the East 1% of oxide being converted to carbide. considered basically a function of temperature. This means that the temperature must normally be raised from 1500 C. to 2000 C. to provide the driving force ti'or solid-solid difiusion) to finish the reaction. Also. UO -C reaction 13 a solid-solid reaction {to obtain a solid product). Complete reaction requires perfect intimate contact between reactants, and this is seldom achieved in actual practice. Therefore, solid-solid diffusion limits the reaction completion. The purpose of the nickel additive in forming a liquid phase is to provdie a transfer media through which oxide and carbon can react during the finishing portion of the reaction. Thus the finishing temperature is lowered by the process described in this invention.

While the invention has been described in particular reference to producing DC from U03, our experiments thow that the invention has other applications for protlIlClng :naterial low in oxygen content and high in tensity in the carbothermic process. Examples are for conterting the oxides of Nb, Ta, Hf, Mo, W, Y, V, Zr, U-Pu tilfld mixtures of these oxides with uranium oxide to obtain their carbides with equally excellent characteristics.

in addition. the process described supra is equally efiective in producing refractory nitrides from mixtures of tarbon, nitrogen and oxides such as TiO ZrO HfO WW-305 and NbO tlthough a preferred embodiment of the invention has been described as required, the scope of this invention is tieiined in the following claims.

11 claim:

it. in a process for forming improved uranium carbide the steps comprising:

.zinixing powdered uranium oxide, carbon and nickel,

tleating said mixture in a reducing atmosphere to a temperature suiiicient for melting said nickel as a afluxing agent, and

raising the temperature of the heated mixture to a value "cient to vaporize the nickel, but less than the itporization temperature of the uranium carbide. t method as in claim 1, wherein said mixture is heated for several hours to a temperature in the range if 1350-1550 C. and wherein the temperature is subtequently raised to a temperature in the range of 1550"- i C. ior several more hours.

.lileferences Cited .lll lITED STATES PATENTS V1967 Strausberg 23349 1967 Beucherie et a1. 23349 tIIllTI-IER REFERENCES Lt tlRL D. UUARFORTH, Primary Examiner.

Jill. i. MCGREAL. Assistant Examiner. 

